Gas metal arc welding (GMAW) is a welding process in which an electrical arc between a filler metal and a work piece heats the filler metal and the work piece and welds them together. The filler metal in the GMAW process is usually a consumable electrode which is fed into the process as fast as it is consumed. The electric current passes through the electrode and the electrical arc is formed between the tip of the consumable electrode and the metal of the work piece. The GMAW welding process can be used to join two pieces of sheet metal together, as well as in many other applications. An example of a welding gun and an arrangement for GMAW is schematically shown in FIG. 1. A consumable welding electrode 14 is fed into the welding process through a welding gun 10. Electrode 14 is melted by an electrical arc 18 established between the electrode and the work piece consisting of metal sheets 11 and 13. Externally supplied gas, such as Ar, CO2 or mixtures thereof, enters the welding process through a gas nozzle 12 in welding gun 10 and shields the arc, the tip of the electrode and the pool of molten metal 15 by forming a gas shield 16. The advantages of the GMAW process are the high quality weld that can be produced faster and with very little spatter and loss of alloying elements due to the gas shield and a stable electrical arc. The consumable electrode in FIG. 1, which is melted by the electrical arc, is transported by the arc to the work piece to serve as a filler metal. The arc produces the heat for the welding process and is maintained by the electron flow between a cathode (positive terminal) and an anode (negative terminal). In the GMAW context both the consumable electrode and the work piece can function as a cathode or an anode.
The electrical power for arc welding is obtained in two different ways. One of the ways is to generate it at the point of use, the other way is to convert it from available power from the utility line. The power conversion can involve a transformer converting a relatively high voltage from the utility line to a liner voltage for alternating current welding. Or it can involve a transformer to lower the voltage, following by a rectifier changing the alternating current to direct current for direct current welding. One of the advantages of the alternating current is cathode-related cleaning (sputtering) which removes refractory oxides from the joint surfaces, providing superior welds. In such a case, argon is the inert gas of choice for manual welding whether used with direct or alternating current.
The growing demand for increased electric arc welding productivity calls for continuing efforts to reduce welding time while improving productivity, especially in robotic welding applications. In order to operate a welder at its maximum capacity, a consumable electrode should be able to form good welding beads at a maximum possible travel speed without sacrificing the quality of the resulting weld. One of the ways to increase productivity is to increase the deposition rate and travel speed for a given weld size. On the other hand, it often happens that an increase in a travel speed leads to an increase of the number of welding defects.
One of the ways to increase deposition rates and travel speed without sacrificing other welding parameters is to change the geometrical structure and composition of consumable electrodes, which are often used in the form of wires. A wire electrode can be a solid electrode, as shown in FIG. 2A, or a cored electrode comprised of an external sheath and an internal core, as shown in FIG. 2B. One of the principles a developer uses in designing such electrodes for higher deposition rates is to increase electrical resistance of the wire electrode. The increased electrical resistance leads to increased heat generation and higher melting rate, leading to the faster speed of melting of the wire and to the desired higher deposition rates.
The known cored wire electrodes are usually classified as metal core wires and flux core wires. Cored wires are typically comprised by a metal or flux powder compacted into a solid granular-type core. Manufacturing of the cored wires usually involves forming, filling and then drawing or rolling the wire. A steel sheath is bent into a U-shape strip, then a predetermined amount of a metal powder, for example, iron powder, is fed into the U-shaped strip. The subsequent forming and drawing processes enclose the powder in the sheath and compact the wire to its final shape and size. Because of the compacted metal powder in the core of the wire, its electrical resistance to the flow of current is greater than that of a solid wire. Consequently, the deposition rates of metal cored wires are much higher than those of the solid wire electrodes. On the other hand, manufacturing of the metal core electrodes can be rather complex, since the powdered mixture is fed into a formed tube of a metal strip moving at high speed. Precise control of this process becomes very important to maintaining high quality wire manufacture, because sometimes a flux or powder dispenser has difficulties ensuring consistent filling of the tube.
One of the main characteristics of the metal core wires with compacted powdered metal in the core is the core fill percent. Variation of the core fill percent in turn causes variations of spatter during the GMAW process. Small variations of the ionizing potential caused by the changes in the core fill percent disturb the electric arc and cause undesired stutter during welding. Therefore, reducing the variations of the core fill percentage and the sputter has been an important consideration in designing the structure and composition of consumable cored electrodes.